This invention relates generally to sub-surface formation boundary detection and more specifically to method and apparatus for such detection using borehole telemetry apparatus.
It is known that shale in natural formations may contain significant amounts of hydrocarbon. It is further well known that significant amounts of hydrocarbon can be recovered by boring holes into the formation and using so-called fracking techniques. Such beds may extend generally horizontally, and be relatively thin in vertical extent. U.S. Pat. Nos. 6,280,000 and 6,425,448 describe examples of such drilling and show particular patterns of holes to drain methane from a coal formation. In the boring of such holes, method and means are needed to steer the drilling progress, so as to remain in the bed and, to the extent possible, bore a straight hole such that up and down variations in the borehole path are minimized.
Conventional or current boring, or drilling, operations use some sort of measure-while-drilling (MWD) apparatus. Such an apparatus generally includes inclination and direction sensors, various logging sensors to assist in determining that the borehole trajectory remains in the underground formation and a communication means to transmit data to the surface so that the necessary control operations to control the drill string path can be performed. Typical inclination sensors include accelerometers to sense the earth's gravity field. The most commonly used direction sensors are magnetometers to sense the earth's magnetic field although gyroscopic sensors may be used in some circumstances. Logging sensors may include conventional resistivity sensors based in the low-megahertz frequency range, total gamma ray sensors and focused gamma ray sensors. In current practice, the only sensors that can provide reliable information as to whether or not the drilling apparatus is within or out of the selected formations are the various gamma ray sensors. Theses sensors generally have a very short range, perhaps only a few inches, and thus the drill bit may already be out of the selected formation by the time that gamma ray sensors provide an indication of such a condition. Given this limitation, such boreholes may have considerable variation in inclination as the path of the drill bit is steered. Further, conventional resistivity tools would increase the length of the bottom hole assembly at the bottom of the drill string and would increase the cost of drilling. While certain resistivity apparatus and methods are used to steer the drilling apparatus in order to maintain the borehole in a desired geological bed, none of these is similar to or has the advantages of the present invention described below.
There is a need for improved sensing method and means that can efficiently detect the boundary of the selected formation, such as shale formation, at a considerably greater depth of investigation around the borehole and most desirably one that can provide some indication of the conditions out ahead of the bit so as to permit correction of the drill path with reduced variation in inclination.
In the measure-while drilling (MWD) process for drilling, the borehole telemetry technique of choice is the electric field technique that involves direct injection of electric current into the surrounding formation at a point below an insulating gap in the generally conducting steel drill string. This injected current flows out into the formation and develops a detectable electric voltage between a remote contact to the earth and the drill string at the surface of the earth. Examples of such apparatus are disclosed in U.S. Pat. Nos. 5,130,706, 5,883,516, 6,188,223 and 6,396,276. It has been observed experimentally, and confirmed analytically, that when the drill bit is employed in a coal seam the apparent driving-point impedance, defined as the ratio of the output voltage to the output current, seen at the output stage of an electric field borehole telemetry apparatus decreases as the drill bit below an insulating gap approaches a coal seam boundary and penetrates into an adjacent rock layer. Further, it has been observed experimentally and confirmed analytically that the received signal strength at the surface of the earth increases for the same approach to and penetration into an adjacent rock layer.